An NOx selective catalytic reduction catalyst for reducing NOx in exhaust gas may be arranged in an exhaust passage of an internal combustion engine. In general, the NOx selective catalytic reduction catalyst is formed such that at least one kind of active component, such as Fe, Cu or the like, exhibiting a selective catalytic reduction property with respect to NOx is supported in the pores of zeolite by ion exchange. In the NOx selective catalytic reduction catalyst, the NOx in the exhaust gas is selectively reduced by the supply of ammonia. Here, fuel in the exhaust gas may adhere to the NOx selective catalytic reduction catalyst and the fuel thus adhered covers the active component, thereby leading to a fuel poisoning state where the NOx reduction ability of the NOx selective catalytic reduction catalyst decreases. This fuel poisoning state causes a decrease in the NOx removal or purification rate of the NOx selective catalytic reduction catalyst. For that reason, there has been developed a technology in which the temperature of the NOx selective catalytic reduction catalyst, which has fallen into the fuel poisoning state, is raised, thereby eliminating the poisoning state, for example, as shown in Japanese patent laid-open publication No. 2009-41437. In addition, with respect to, not the NOx selective catalytic reduction catalyst, but the NOx storage reduction catalyst, there has been disclosed, for example in Japanese patent laid-open publication No. 2005-30272, a technology of limiting an amount of fuel in exhaust gas so as to prevent the occurrence of such a fuel poisoning state, similarly, in view of the decrease in the NOx purification rate due to fuel poisoning.
Further, in an exhaust gas purification system having a selective catalytic reduction filter (SCR filter) which is formed by carrying an NOx selective catalytic reduction catalyst on a filter, filter regeneration processing is carried out in which fuel is supplied into exhaust gas in order to oxidize and remove particulate matter deposited on the SCR filter, so that the temperature of the exhaust gas is caused to rise in an oxidation catalyst disposed on the upstream side of the SCR filter (for example, see Japanese patent laid-open publication No. 2014-148908). When such filter regeneration processing is carried out, fuel is supplied into the exhaust gas, and hence, there arises a possibility that some amount of fuel also may flow into the SCR filter, thereby causing fuel poisoning of the NOx selective catalytic reduction catalyst in the SCR filter.